1. Field of the Invention
The present invention relates to a piezoelectric element, an ultrasound probe, and an ultrasound imaging apparatus having the ultrasound probe.
2. Description of Related Art
An ultrasound probe in an ultrasound imaging apparatus to be utilized in the medical field has a piezoelectric element including a piezoelectric composition and an electrode that applies a voltage to the piezoelectric composition. In the ultrasound imaging apparatus, the piezoelectric composition is vibrated by an electrical signal to thereby generate ultrasound to be transmitted. The ultrasound reflected in a subject is received by the ultrasound probe. Thus, the ultrasound imaging apparatus can obtain an ultrasound image of the subject.
The ultrasound probe preferably includes a piezoelectric composition having a high relative permittivity, a coercive electric field and an electromechanical coupling coefficient from the viewpoint of realizing an ultrasound probe having a high sensitivity to ultrasound. As an example of such an ultrasound probe, an ultrasound probe is known which includes a piezoelectric composition having a perovskite structure including zirconium in a predetermined composition (see, for example, Japanese Patent No. 5063606). The piezoelectric composition of the ultrasound probe has a relative permittivity of 4,000 or more and a coercive electric field of about 5.3 to 10 kV/cm. The relative permittivity of the piezoelectric composition described in Japanese Patent No. 5063606 here means a value measured at a sufficiently low frequency (usually 1 kHz) after a polarization treatment, namely, a relative permittivity (εT) in a free state.
As another example of the ultrasound probe, a piezoelectric element is known which includes a piezoelectric composition being a BiScO3-based solid solution (see, for example, Japanese Patent Application Laid-Open No. 2006-188414). The relative permittivity of the piezoelectric composition of the piezoelectric element is also measured at a low frequency.
There is a case where a piezoelectric composition having a small thickness is used, for example, a case where an acoustic back layer acoustically coupled to a piezoelectric element is included, a case where ultrasound having a high center frequency (for example, center frequency: 7 MHz or more) is used, or a case where a piezoelectric element is layered. In such a case, there is demanded an ultrasound probe that can be driven at a high driving voltage and that has a higher sensitivity to ultrasound than a conventional ultrasound probe, from the viewpoint of a further increase in performance of an ultrasound imaging apparatus.
In the above case, the piezoelectric composition is in a substantially bound state. That is, the piezoelectric composition of an actual ultrasound probe is fixed to other member (for example, acoustic back layer) by an adhesive or the like, and is thus in a bound state to some extent. Furthermore, when the frequency of ultrasound is in the vicinity of the antiresonant frequency or is higher than the antiresonant frequency, the piezoelectric composition is in a substantially bound state. Therefore, not relative permittivity εT (hereinafter, also referred to as “free relative permittivity”) in a free state after a polarization treatment, but relative permittivity εS (hereinafter, also referred to as “bound relative permittivity”) in a bound state after a polarization treatment is important for the design of an ultrasound probe.
The design of a probe in consideration of a free relative permittivity cannot sometimes provide any ultrasound probe having a piezoelectric element having a small thickness, the probe realizing desired piezoelectric characteristics and having a sufficient sensitivity to ultrasound. As is clear from the above PTLs, an ultrasound probe has been conventionally known which includes a piezoelectric element having a piezoelectric composition focused on a free relative permittivity.
While an ultrasound probe is preferably high in electromechanical coupling coefficient with respect to a vibration mode in a direction (longitudinal direction) parallel with the polarization direction (electric field direction), it is preferably low in electromechanical coupling coefficient with respect to a vibration mode in a direction (lateral direction) perpendicular to the polarization direction (electric field direction). As is clear from Japanese Patent Application Laid-Open No. 2006-188414, a piezoelectric component for a low frequency, focused on the electromechanical coupling coefficients (kp, k31) in the lateral direction, such as a piezoelectric speaker or a piezoelectric pump, has been conventionally known as a piezoelectric element having a PMN-PZT-based piezoelectric composition partially replaced with BiScO3. Any piezoelectric element including the piezoelectric composition, however, is not known at all which is designed in consideration of bound relative permittivity ε33S and electromechanical coupling coefficient k33 in an ultrasound region (PMN and PZT represent Pb(Mg1/3Nb2/3)O3 and Pb(ZrTi)O3, respectively).
Furthermore, no ultrasound probe has been found, which is focused on a bound relative permittivity, and the electromechanical coupling coefficient or the effective electromechanical coupling coefficient in the longitudinal direction and which includes a piezoelectric composition sufficiently high in all of a bound relative permittivity, a coercive electric field and an effective electromechanical coupling coefficient. Accordingly, a piezoelectric composition having a small thickness cannot sometimes impart a sufficient sensitivity to ultrasound.